or brown algae
, (singular: alga) is a large group of mostly marine
, including many seaweeds
of colder Northern Hemisphere
waters. They play an important role in marine environments both as food, and for the habitats they form. For instance Macrocystis
, a member of the Laminariales
, may reach 60 m
in length, and forms prominent underwater forests
. Another example is Sargassum
, which creates unique habitats in the tropical waters of the Sargasso Sea
. This is one of the few areas where a large biomass of brown algae may be found in tropical waters
. Many brown algae such as members of the order Fucales
are commonly found along rocky seashores. Some members of the division are used as food for humans.
Worldwide there are about 1500-2000 brown seaweed species.
Brown algae belong to a very large group, the Heterokontophyta, a eukaryotic group of organisms distinguished most prominently by having chloroplasts surrounded by four membranes, suggesting an origin from a symbiotic relationship between a basal eukaryote and another eukaryotic organism. Most brown algae contain the pigment fucoxanthin, which is responsible for the distinctive greenish-brown color that gives them their name. Brown algae are unique among heterokonts in developing into multicellular forms with differentiated tissues, but they reproduce by means of flagellate spores, which closely resemble other heterokont cells. Genetic studies show their closest relatives to be the yellow-green algae.
The plants are filamentous, macroscopic or microscopic some polyiphonous. Some form crusts, cushions or are hollow and others grow to form large leathery fronds. The cells of most browns are connected by pores. They bear a unilocular (one holed) sporangium.
Phaeophyta evolved from the phaeothamniophyceae between 150 & 200 million years ago. Claims that earlier (Ediacaran) fossils are brown algae have since been dismissed.
The linages of brown algae diverged in the following order, from oldest to youngest: Dictyotales; Sphacelariales; Cutleriales; Desmerestales; Ectocarpales; Laminarales; Fucales.
Their occurrence as fossils is rare due to their generally soft-bodied habit, and scientists continue to debate the identification of some finds. Other algae groups, such as the red algae and green algae have a number of calcareous members, which are more likely to leave evidence in the fossil record than the soft bodies of the brown algae. Miocene fossils of a soft-bodied brown macro algae, Julescrania, have been found well-preserved in Monterey Formation diatomites, but few other certain fossils, particularly of older specimens are known in the fossil record.
This is a list of the orders
in the class Phaeophyceae:
A few species, such as Botrydium stoloniferum, are placed incertae sedis, or of uncertain position, as to order in this classification scheme.
The life cycle shows great variability from one group to another. However the life cycle of Laminaria
consists of the diploid
generation, that is the large plant well know to most people. It produces sporangia from specialised microscopic structures, these divide meiotically (meiosis
) before they are released. As they are haploid
there are equal numbers of male and female spores. With the exception of the Fucales all brown algae have a life cycle which consists of an alternation between morphologically haploid and diploid plants referred to as Monomorphic. A dimorphic life cycle consists of an alteration between dissimilar haploid and diploid plants.
Brown algae have adapted to a wide variety of marine ecological niches including the tidal splash zone, rock pools, the whole intertidal zone and relatively deep near shore waters. They are an important constituent of brackish water ecosystems, and four species survive in fresh water. Most phaeophyceae are intertidal or upper littoral, and they they are predominantly cool and cold water organisms that benefit from nutrients in up welling cold water currents and inflows from land; Sargassum
being a prominent exception to this generalisation.
Brown algae growing in brackish waters are almost solely asexual.
Brown algae have a value between , in contrast with red algae and greens. This reflects their different metabolic pathways.
They have Cellulose walls with alginic acid; fucoidin also important in amorphous section of cell walls. A few species (of Padina) calcify with aragonite needles.
Druehl, L.D. 1988. Cultivated edible kelp. in Algae and Human Affairs.
Lembi, C.A. and Waaland, J.R. (Editors) 1988.ISBN 0 521 32115 8.